Charged current DIS with polarised e ± beams at HERA. Alex Tapper

Transcription

1 Charged current DIS with polarised e ± beams at HRA Alex Tapper

2 The HRA accelerator e± p 27.5 GeV 920 GeV s=320 GeV Page 2

3 Longitudinal polarisation at HRA Longitudinal polarisation of the lepton beam at HRA: new at HRA II = N R "N L N R +N L Transverse polarisation builds up naturally through synchrotron radiation (Sokolov-Ternov effect) Build-up time around 40 minutes at HRA Spin rotators flip transverse polarisation to longitudinal before interaction regions and back afterwards Polarisation measured by two independent Compton polarimeters (+ new one being commissioned currently) Page 3

4 The H1 and ZUS detectors LAr calorimeter (45000 cells) M " () 12% HAD " () = = 50%! 1%! 1% DU calorimeter (6000 cells) M! () 18% HAD =! () = 35% Page 4

5 Deep inelastic scattering at HRA Two deep inelastic scattering processes: Neutral current: exchange of γ or Z 0 See talk by V. Chekelian Charged current: exchange of W ± The topic of this talk Q 2 = "q 2 = "(k " k #) 2 Q 2 is the probing power x is the Bjorken scaling variable y is the inelasticity x = Q 2 2p"q s = (p + k) 2 y = p"q p"k Q 2 = x " y " s Page 5

6 Charged current DIS at HRA CC e + p cross section: Sensitive to density of d quark d 2 " CC (e + p ) dxdq 2 = G 2 F 2# ( 2 M W ) 2 u + c + (1$ y) 2 (d + s) M 2 W +Q 2 [ ] CC e - p cross section: 2 " (x, Q ) / x d 2 " CC (e # p ) dxdq 2 = G 2 F 2$ ( 2 M W ) 2 u + c + (1# y) 2 (d + s ) M 2 W +Q 2 [ ] Sensitive to density of u quark lectron/positron-proton collisions probe different quark content of proton Big difference in cross section magnitude u-quark density larger than d-quark d-quark contribution suppressed by helicity factor (1-y) 2 Page 6

7 Polarised charged current DIS Polarisation is asymmetry of helicity states Helicity = chirality (neglecting masses) Can use polarised beams to directly test chiral structure of the Standard Model Standard Model weak interaction left-handed only LH particles (RH anti-particles) interact = N R "N L N R +N L CC cross section modified by : e " ± p e CC ( ) = (1± ) # " ± p CC ( = 0) Polarisation scales =0 cross section linearly - clear and large effect at HRA Standard Model predicts zero cross section for =+1(-1) in e -(+) p scattering Page 7

8 Data samples: luminosities and polarisations Data sample H1 ZUS <0 >0 <0 >0 e + p =-0.40±0.01 =+0.34±0.01 =-0.41±0.01 =+0.32±0.01 L = 20.7 pb -1 L = 26.9 pb -1 L = 11.5 pb -1 L = 12.3 pb -1 e - p =-0.27±0.01 =+0.37±0.02 =-0.27±0.01 =+0.33±0.02 L = 68.6 pb -1 L = 29.6 pb -1 L = 78.8 pb -1 L = 42.7 pb -1 Page 8

9 Charged current events neutrino e ± p Neutrino escapes undetected CC candidate events selected using missing T Topological cuts to remove non-ep backgrounds ep backgrounds estimated by MC and subtracted Typically around 1% Page 9

10 Detector calibration Only the hadronic final state available to reconstruct the event - xcellent understanding of detector effects necessary - Use high statistics NC DIS samples to calibrate detectors Page 10

11 Dependence on Clearly demonstrate linear dependence on Consistent with SM predictions Direct sensitivity to W R next slide Page 11

12 W R mass limit Linear fit to dependence gives cross section limit at P=-1(+1) for e +(-) p Assume g R =g L and ν R is light Cross section limit at P=-1(+1) for e +(-) p gives lower limit on mass of W R M WR > % CL (from H1 e - p) M WR > % CL (from ZUS e - p) M WR > % CL (from H1 e + p) Page 12

13 Single differential cross sections Cross sections as a function of Q 2, x and y Clear difference between different polarisations Independent of kinematic variables as predicted by the SM Combine polarised samples to give unpolarised cross sections Clear difference between e - p and e + p as predicted by SM Page 13

14 Double differential cross sections Reduced cross section = PDF content Measured with much higher precision than ever before in charged current DIS Input to QCD and W fits see talk by Y. Ri Page 14

15 Summary and future prospects Charged current DIS with longitudinally polarised lepton beams measured first e + p results published (H1: PLB 634 (2006) 173 ZUS: PLB 637 (2006) 210) e - p results preliminary Measurements in good agreement with Standard Model Set limit of M WR >208 GeV Still more data to analyse in e - p scattering More data to come in e + p scattering Higher precision measurements to come Page 15